In this paper the factors intertering with development and individuation of twins are described. The problems for the mother in her relation to a twin are discussed, as well as the 'twinning reaction' or the inter-identification of twins, the strong and mostly very positive relationship of twins and their rivalry hidden behind that relation, the role playing in twins and the facts causing that mechanism. The circumstances forcing soine siblingrelations into a semi-twinrelationship are stressed. Attention is given to the influence of twin-interaction on the development of the 'folie d deux.'